We are studying fatty acid profile bone marrow in hemopoietic (red) and aplastic (yellow) marrow. Preliminary results suggest that a shift in fatty acid composition of marrow adipose tissue in yellow marrow may render the adipose cells more stable such that when hemopoiesis expansion occurs, these cells cannot be replaced by blood forming elements. We are further studying the response of marrow adipose cells to starvation and hemopoietic stimulation as compared to adipose cells in extramedullary sites using electron microscopy. Preliminary results indicate a distinct difference between these two types of tissue suggesting that their primary functions are completely different. We are studying the delivery of red cells into circulation and the mechanisms that control this process. We are using carbonyl iron particles, injected intravenously. This results in breakdown of the blood-marrow barrier and consequently large numbers of nucleated red cells are released into circulation. We are using implantation of spleen pieces in mice with Friend's leukemia in order to determine the integrity of hemopoietic stroma in leukemic states. Successful regeneration of splenic implants depends on the integrity of stroma.